1. Field of Use
The present invention relates generally to the field of antennas. More particularly, the present invention concerns scanning antennas. Specifically, a preferred embodiment of the present invention is directed to a photoinduced coupling antenna. The present invention thus relates to antennas of the type that can be termed photoinduced coupling scanning antennas.
2. Description of Related Art
Within this application several publications are referenced by arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the invention and illustrating the state of the art.
Vehicle crashes represent a significant public health hazard as well as a cause of significant economic loss each year. Therefore, there has been a long felt need for an inexpensive crash avoidance system for use in automobiles, aircraft and other vehicles.
The National Highway Traffic Safety Administration (NHTSA) has identified Autonomous Intelligent Cruise Control (AICC) and similar autonomous crash avoidance systems as precursors to fully automated driving in the proposed future Automated Highway System. Highway crashes are the sixth leading cause of death in the USA, and the major cause of death for people below the age of 25. A recent NHTSA report gives the costs associated with the 44,531 deaths, 5.4 million injuries, and 28 million damaged vehicles in 1990; the losses are estimated to be $137.5 billion in lost wages and other direct costs. The economic loss from traffic crashes represents greater than 2% of the U.S. GNP, and results in nearly 2 billion hours of lost time and 7.5 million liters of wasted fuel each year. Aircraft, and other vehicle, crashes also represent a significant economic loss each year.
Crash avoidance systems (CAS) for highway vehicles are conventionally designed to be a countermeasure to one or more classes of recognized crash types. Crash avoidance systems generally fall into three categories: near obstacle detection systems (NODs), forward looking radar (FLR) systems, and wide angle imaging systems for all weather and night vision (AWNV).
The clear choice of wavelength for FLR and AWNV sensors is the millimeter wavelength (MMW) range. The European frequency allocation for such sensors is 76 to 77 GHz. The Japanese frequency allocation for such sensors is currently 59 to 60 GHz. The U.S. frequency allocation for such sensors, while still under discussion, has tended to be around 76 to 77 GHz, although 94 GHz has also been discussed. The electronic and signal processing parts of FLR and AWNV systems are considered to be essentially developed and ready for mass production.
For example, millimeter wavelength transceiver electronic packages for use in conjunction with the crash avoidance systems for vehicles such as, for example, automobiles are already commercially available. A specific example of such a commercially available transceiver electronic package is Litton's millimeter wavelength transceiver..sup.(2)
However, an inexpensive scannable millimeter wavelength antenna is not yet commercially available for use with such crash avoidance systems. As a practical economic matter, the phase shifting element solution used for prior art seeker applications cannot be adopted for use in a commercial vehicle crash avoidance system because of the extremely high cost of the individual phase shifting elements that are a part of such seeker applications, (i.e., from approximately $2,000 to approximately $10,000). Further, the phase shifting element solution used for prior art seeker applications cannot be adopted for use in a commercial vehicle crash avoidance system because of the very high cost of the skilled hand labor required for the assembly of such a phased array antenna.
An IEEE workshop in May 1994.sup.(1) on millimeter wavelength technology for automobiles identified the millimeter wavelength scanning antenna as a key element needed to complete an economically viable automobile crash avoidance system. However, of more than 30 existing antenna technologies previously studied, none satisfies the full range of required parameters, especially the possibility of being mass produced at very low cost.
A millimeter wavelength scanning antenna that is economically feasible for use in automobiles would probably be feasible for use in more expensive vehicles such as, for example, aircraft. A commonly accepted cost for an economically feasible forward looking millimeter wavelength antenna for an automobile is presently approximately $50. Clearly, the existing antennas that are used for prior art seeker applications cannot be manufactured at such a low cost. Therefore, there has been a long felt need for a low cost millimeter wavelength scanning antenna.
The availability of a low cost millimeter wavelength scanning antenna would make an inexpensive automobile crash avoidance system a commercial reality. Such a low cost millimeter wavelength scanning antenna could also be used to provide an inexpensive aircraft, or other vehicle, crash avoidance system.
The below-referenced U.S. patent discloses embodiments that are satisfactory for the purposes for which they were intended but which have certain disadvantages. The disclosure of the below-referenced prior United States patent in its entirety is hereby expressly incorporated by reference into the present application.
U.S. Pat. No. 5,305,123 discloses a light controlled spatial and angular electromagnetic wave modulator. In embodiments disclosed in the above-referenced prior patent, periodic perturbations of the complex dielectric field in the surface of a semiconductor material induced by an optical control pattern cause electromagnetic waves to be coupled out of the semiconductive material in a particular direction depending upon the period of the perturbations. Rapid variations in the period of the perturbations can be induced by controlling the optical control pattern. By rapidly changing the period of the perturbations, (i.e., the grating period induced by the optical control pattern), can be used to control the direction of beam steering and forming can be achieved.
Light can change the complex refractive index n of a semiconductor material. Specifically, n=n (I) where n=n'+in" (where n' and in" are the real and imaginary parts respectively and and I is the intensity of the optical wave. The mechanics of this phenomenon is based on fundamental Drude theory. See T. S. Moss, "Optical Properties of Semiconductors," Butterworths, London (1959).
The intensity I of an optical wave can change the complex refractive index of Si, GaAs, InGaAsP and other semiconductors in the microwave range (1 mm-1 cm) and infrared (IR) range (1.0.mu.-100.mu.). See I. Shih, "Photo-Induced Complex Permittivity Measurements of Semiconductors" 477 SPIE 94 (1984) (microwave range) and B. Bennett, "Carrier-Induced Change in Refractive Index of InP, GaAs, and InGaAsP" 26 IEEE J. Quan. Elec. 113 (1990) (IR range) incorporated herein by reference.
The prior art shows light induced modulation of both the real and imaginary parts of the refractive index. The real part controls phase and the imaginary part controls amplitude of the modulated electromagnetic field. The real part is primarily responsible for changes in IR waves and the imaginary part for changes in millimeter waves (MMW). This effect is described by Drude theory and involves carrier induced changes in the complex permittivity of metals and semiconductors when illuminated by light. Light increases the density of free carriers in the material.
Based on this effect, devices which change the phase of lightwaves by illumination of semiconductors with other light have been developed. Specifically, optical phase modulators have been employed. In the state of the art, however, it is shown possible to modulate the material at only one point. This type of limited modulation is discussed in a recent article by Z. Y. Cheng and C. S. Tsai "Optically Activated Integrated Optic Mach-Zender Interferometer on GaAs," 59 Appl. Phys. Lett. 1991. It would be beneficial to employ a device that can modulate an EM field at more than one point, particularly to modulate the material in two dimensions (2D) and potentially three dimensions (3D).
At the same time, optically controlled spatial light modulators (SLM) based on semiconductor materials have been used. In optically addressed SLMs, the semiconductor plays a transport role, such that changes in the semiconductor material affect an adjacent layer of electro-optic (EO) material which in turn affects an EM wave propagating through the EO material. The effectiveness of this type of modulator is low. It should be mentioned that these devices are limited to controlling the visible range only.
These SLM devices transmit or project some 2D pattern which can be transmitted through an optical wave. Other types of devices that are of interest, however, transmit EM waves in a particular direction in the microwave region without moving parts. Such devices are called phased array antennas.
A phased array is a network of radiating elements, each of which is usually non-directive but whose cooperative radiation pattern is a highly directed beam because constructive interference occurs between radiating elements. Whereas previous radar antennas had to be mechanically steered for beampointing, the phased array antenna achieves the same effect electronically by individually changing the phases of the signals radiating from each element. Narrow angular band beams can be formed by simply driving each element of the array with an appropriately phased signal. Moreover, electronic steering is much faster and more agile than mechanical beam steering and can form several beam lobes and nulls to facilitate multiple target tracking or other functions such as anti-jamming.
The flexibility of electronic steering afforded by phased array radars, however, comes at the cost of individual control of each element. The N elements of the antenna are driven with the same signal but each with a different phase. In practice, a single signal is equally split into N signals to feed the elements, and a phase shifting network, such as those using ferrites or diodes, is provided for individual phase control of each element. For large arrays (i.e., N&gt;100), the complexity of the power splitting network and the cost of providing N phase shifters can become quite high, not to mention the bulkiness of the necessary waveguide plumbing. Moreover, for very large arrays, the computation required to calculate the array phase distribution for a desired radiation pattern is a serious burden. These constitute the most serious drawbacks of conventional phased array radar systems.
Phased array antenna theory is based on Fourier optics in general and the theory of diffraction gratings in particular. It is well known from Fourier optics that the optical beam is diffracted in a particular direction if the phase difference between the particular optical rays is a multiple of the wavelength of the optical beam.
The phase synchronized condition has the form m.lambda.=.LAMBDA.sin.theta., where .LAMBDA. is the grating constant, .theta. is the angle of diffraction, and m is the integers 0, .+-.1, .+-.2 . . . From this equation we obtain the following equation ##EQU1## which is the well known grating equation. If electrically controlled, a phase shift of m.lambda. can be introduced between different antennas thereby causing constructive interference in one direction, which results in antenna directionality. This effect can be used in both a transmitter and a receiver.
Exactly the same principle is used in conventional phased array antennas where illuminated points of the gratings are replaced by elementary antennas. See M. I. Skolnik, "Introduction to Radar Systems", McGraw Hill N.Y. (1980) incorporated herein by reference.
Using Equation A two basic disadvantages of phased array antenna systems are made apparent: (1) the periodic structure has a discrete point-type profile. This means that many diffraction orders are generated; only one order is desired, and the remaining orders reduce efficiency of the system; (2) the number of elementary antennas is limited by size and complexity. As the frequency of the microwaves increase (beyond 60 GHZ), the density of packaging of individual elements and phase shifters limits the feasibility of such an antenna. Also, having individual emitters fixed in space precludes the antenna from being used for different frequencies. At the receiver end, such an antenna has limited bandwidth capability (due to the fixed elements).
For IR beam steering the packaging of individual phase shifters is virtually impossible and an electronically controlled spatial light detector is limited to very narrow angular bandwidth. See F. Vasey et al., "Electro-optic AlGaAs Spatial Light Deflector/Modulator Based on a Grating Phased Array" 58 Appl. Phys. Lett. 2874 (1991) incorporated herein by reference.